structure? yes.
To answer the OP's question, the answer is yes. In a monocoque structure, just about every panel contributes to torsional stiffness in one way or another. Every bulkhead cover, seat pan, baggage floor, that sort of thing, closes out bays that create torsional stiffness.
Torsional stiffness is especially challenging in the cockpit area where the main fuselage 'tube' is cut open to create the cockpit. If you want to visualize this, take an empty paper towel tube, and cut a rectangular hole in it roughly in proportion to what your RV-6,7,8,9 cockpit opening is. Now twist the tube and watch.
Lots of modifications can reduce or even cripple torsional strength. A common one is the access panel that people put in the bulkhead of RV-8s between the instrument panel area and the baggage compartment. If that access panel closes with lots of screws along the perimeter of the cut-out, great. If it just has a latch or two along one edge, not good. Another approach besides lots of screws around the perimeter would be to put a heavy doubler around it -- the bulkhead contributes to torsional stiffness by maintaining the cross section shape. A doubler will help do that - it basically prevents the compromised bulkhead from buckling.